This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-207322, filed Jul. 7, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a printed wiring board having a via for electrically connecting a plurality of wiring layers and a method of manufacturing printed wiring board.
2. Description of the Related Art
For example, a semiconductor package used for electronic apparatus such as a portable computer or the like is mounted on a printed wiring board. This printed wiring board has an insulating layer made of synthetic resin and a plurality of wiring layers. The insulating layer has a first surface and a second surface located on the opposite side of this first surface. The wiring layers are laminated on the first and second surfaces of the insulating layer, respectively. The wiring layers are formed so as to correspond to the predetermined circuit pattern, signal pins used for inputting and outputting for a semiconductor package and power source pins are electrically connected to these wiring layers.
Recently, as for a semiconductor package, the number of signal pins is much increased along with the higher packing density and higher performance, and there is increasing tendency to density of the signal pins. Therefore, for example, a wiring layer of a printed wiring board connected to a signal pin has to be arranged between pins arranged in a high density. As a result, a pattern width of a wiring layer itself becomes extremely narrow as well as a pitch of the wiring layer is finely divided.
On the other hand, this kind of printed wiring board has a via for electrically connecting a plurality of wiring layers which are separated by an insulating layer being interposed therebetween. This via is formed on the insulating layer in the steps of manufacturing a printed wiring board.
A conventional printed wiring board is manufactured according to the following procedure. First, a copper clad laminate is prepared. The copper clad laminate is a laminated layer in which copper foils are laminated on the first and second surfaces of the insulating layer, respectively. After this copper clad laminate is cut into a predetermined shape, a via is formed at a predetermined position of the copper clad laminate. A via is formed by applying a void processing to the insulating layer of the copper clad laminate, and has one end opened on the first surface of this insulating layer and the other end closed by the wiring layer on the second surface of the insulating layer.
When the via is formed on the copper clad laminate, electroless plating is applied to this copper clad laminate. By doing so, a first plating layer which is a groundwork for the conductivity is formed on the copper foil covering the first and second surfaces of the insulating layer and the inner surface of the via. Subsequently, an electrolytic plating is applied to the copper clad laminate, and a second plating layer is laminated on the first plating layer.
Next, the copper clad laminate is covered with an etching resist, and an etching processing is applied to this copper clad laminate. By doing so, the copper foil doubly covered with the first and second plating layers is selectively etched so that the copper foil is to be a fine circuit pattern. When the etching is completed, the etching resist is removed. By doing so, the wiring layers are formed on the first and second surfaces of the insulating layer, respectively. These wiring layers are maintained in a state where these are electrically connected through the first and second plating layers within the via.
However, according to a conventional method of manufacturing a printed wiring board, the first plating layer and the second plating layer are doubly adhered on the copper foil which is to be a wiring layer. Therefore, the thickness of the copper foil is exceeded by the portion of thickness of the first and second plating layers, and the thickness thereof may exceed the thickness limitation of the etching.
Specifically, a pattern width of a wiring layer obtained by etching relates to the thickness of this wiring layer, and usually, the minimum pattern width is made on the order of 10 to 20-fold of the thickness of the wiring layer. Therefore, as making a pitch and a pattern width of the wiring layer finer is pursued, as to the printed wiring board obtained by the above method of manufacture, the wiring layer itself becomes too thick with respect to the pattern width of the wiring layer. As a result, the etching becomes incomplete, the wiring layers adjacent to each other may develop a short circuit, and to make the wiring layer finer cannot be corresponded.
Moreover, a flexible printed wiring board in which a resin film of thermoplasticity is made as an insulating layer is often repeatedly bended or is mounted on electronic apparatus in a state of being bended in a U shape. Therefore, if the plating layers are doubly adhered on the copper foil positioned on the surface which is to be an internal side of the bending out of the flexible printed wiring board, particularly when the flexible printed wiring board is repeatedly bended, the plating layers may be released or a crack may be occurred. This released plating layer may interpose between the adjacent wiring layers and develop a short circuit in these wiring layers. Moreover, the missing of the plating layer causes the wiring layers themselves easily to be broken. Therefore, the reliability of the printed wiring board having the conventional fine wiring layers is low in the viewpoint of an electrical connection.
An object of the present invention is to obtain a printed wiring board in which a wiring layer can be formed to be as thin as possible and can correspond to the tendency of making this wiring layer finer in an affordable way and a method of manufacturing the printed wiring board.
In order to achieve the object, a printed wiring board according to a first aspect of the present invention comprises an insulating layer having a first surface and a second surface located on the opposite side of the first surface; a plurality of wiring layers which are formed so as to correspond to a predetermined circuit pattern and formed by etching metal foils laminated on the first surface and the second surface of the insulating layer, respectively; a via which is formed on the insulating layer and has one end opened on the first surface of the insulating layer and the other end closed by the wiring layer on the second surface of the insulating layer; a first plating layer which continuously covers the inner surface of the via and the wiring layer on the second surface exposed within the via and that portion of the wiring layer which is formed on the first surface and which faces one end of the via; and a second plating layer which is laminated on the first plating layer and electrically connects the wiring layer located on the first surface and the wiring layer located on the second layer by cooperating with this first plating layer.
Moreover, in order to achieve the object, a printed wiring board according to a second aspect of the present invention comprises a laminate which includes an insulating layer having a first surface and a second surface located on the opposite side of the first surface, and a plurality of wiring layers laminated on the first surface and the second surface of the insulating layer and inside of the insulating layer, these wiring layer being formed so as to correspond to a predetermined circuit pattern; a via which is formed on the laminate and has one end opened on the first surface of the insulating layer and the other end closed by the wiring layer inside of the insulating layer; a first plating layer which continuously covers the inner surface of the via and the wiring layer inside of the insulating layer exposed within the via and that portion of the wiring layer which is formed on the first surface and which faces one end of the via; and a second plating layer which is laminated on the first plating layer and electrically connects the wiring layer located on the first surface and the wiring layer inside of the insulating layer by cooperating with the first plating layer.
According to such a configuration, since the first and second plating layers are formed on a portion of the via spanning between the wiring layers, there is no possibility such that the first and second plating layers remain or adhere to the wiring layers. Therefore, the wiring layer can be thinned thereby easily making a pitch and a pattern width of these wiring layers finer.
Moreover, in the case where the insulating layer is made, for example, of a flexible material such as a resin film or the like, the following advantage exists: since the first and second plating layers are not necessary to be laminated on the wiring layers, by employing a rolled copper foil as a metal foil configuring this wiring layer, even if the bending force is repeatedly added to the printed wiring board or even if it is the usage form such that it is bended in a U shape, a break of the wiring layers due to the missing of the plating layers can be prevented. Therefore, the reliability of electrical connection is enhanced as well as the life of the printed wiring board becomes longer.
In order to achieve the object, a method according to the first aspect of the present invention is applied to a printed wiring board including an insulating layer having a first surface and a second surface located on the opposite side of the first surface and a plurality of wiring layers formed so as to correspond to a predetermined circuit pattern, the method comprises: a first step in which the wiring layers are formed on the first and second surfaces of the insulating layer, respectively; a second step in which a via whose one end is opened on the first surface and the other end of which is closed by the wiring layer located on the second surface is formed on the insulating layer; a third step in which a first plating resist covers the second surface of the insulating layer and the wiring layer formed on the second surface; a fourth step in which a first plating layer continuously covers the inner surface of the via, the wiring layer located on the second surface exposed within the via and the wiring layer located on the first surface; a fifth step in which a second plating resist covers a region other than a portion on which one end of the via is opened out of the first surface of the insulating layer as well as the wiring layer located on the first surface; a sixth step in which a second plating layer is laminated on the first plating layer, the wiring layer located on the first surface and the wiring layer located on the second surface are electrically connected through the first and second plating layers; a seventh step in which after electrical connection is completed between the wiring layers, the first and second plating resists are removed; and an eighth step in which the first plating layer exposed on the first surface of the insulating layer is removed along with the removal of the second plating resist.
Moreover, in order to achieve the object, a method of manufacturing a printed wiring board according to the second aspect of the present invention comprises: a fist step in which a laminate including an insulating layer and a plurality of wiring layers is obtained, the insulating layer having a first surface and a second surface located on the opposite side of the first surface, and the plurality of wiring layers being arranged on the first and second surfaces of the insulating layer and inside thereof so as to correspond to a predetermined circuit pattern; a second step in which a via whose one end is opened on the first surface of the insulating layer and the other end of which is closed by the wiring layer inside of the insulating layer is formed on the laminate; a third step in which a first plating resist covers the second surface of the insulating layer and the wiring layer laminated on the second surface; a fourth step in which a first plating layer continuously covers the inner surface of the via, the wiring layer inside of the insulating layer exposed within the via and the wiring layer located on the first surface of the insulating layer; a fifth step in which a second plating resist covers a region other than a portion on which the one end of the via is opened out of the first surface of the insulating layer as well as the wiring layer located on this first surface; a sixth step in which a second plating layer is laminated on the first plating layer, the wiring layer located on the first surface and the wiring layer inside of the insulating layer are electrically connected through the first and second plating layers; a seventh step in which after an electrical connection is completed between the wiring layers, the first and second plating resists are removed; and an eighth step in which the first plating layer exposed on the first surface of the insulating layer is removed along with the removal of the second plating resist.
According to such a method of manufacture, at the time when the first and second plating layers are formed on the inner surface of the via, the second surface of the insulating layer having wiring layer is covered with the first plating resist. Therefore, the first and the second plating layers are not doubly adhered to the wiring layer located on the second surface. Accompanying with it, the other end of the via is closed by the wiring layer. Therefore, the first and second plating layers remain inside of the via and not exposed on the second surface of the insulating layer.
Moreover, at the time when the second plating layer is formed on the inner surface of the via, most of the first surface of the insulating layer other than a portion corresponding to one end of the via is covered with the second plating resist. Therefore, the second plating layer is not adhered to the wiring layer located on the first surface. In addition, since the first plating layer located on the first surface is removed in the final step, the first plating layer does not remain as it is adhered to the wiring layer located on the first surface.
Therefore, the wiring layers located on the first and second surfaces of the insulating layer can be made thinner and a pitch and a pattern width of these wiring layers are easily made finer.
Moreover, in the case where the insulating layer is made, for example, of a flexible material such as a resin film or the like, the following advantage exists: since the first and second plating layers are not necessary to be laminated on the wiring layers, by employing a rolled copper foil as a metal foil configuring this wiring layer, even if the bending force is repeatedly added to the printed wiring board or even if it is the usage form such that it is bended in a U shape, a break of the wiring layers due to the missing of the plating layers can be prevented. Therefore, the reliability of electrical connection is enhanced as well as the life of the printed wiring board becomes longer.
In order to achieve the object, a method of manufacture according to a third aspect of the present invention is applied to a printed wiring board including an insulating layer having a first surface and a second surface located on the opposite side of the first surface and a plurality of wiring layers formed so as to correspond to a predetermined circuit pattern, the method comprises: a first step in which the wiring layers are formed on the first and second surfaces of the insulating layer, respectively; a second step in which a via whose one end is opened on the first surface and the other end of which is closed by the wiring layer located on the second surface is formed; a third step in which a plating resist covers the second surface of the insulating layer and a region other than a portion on which one end of the via out of the first surface of the insulating layer as well as the wiring layers located on the first and second surfaces; a fourth step in which a first plating layer continuously covers the inner surface of the via, the wiring layer located on the second surface exposed within the via and that portion of the wiring layer which is formed on the first surface of the insulating layer and which is other than the plating resist; a fifth step in which a second plating layer is laminated on the first plating layer and the wiring layer located on the first surface and the wiring layer located on the second surface are electrically connected through the first and second plating layers; and a sixth step in which after an electrical connection is completed between the wiring layers, the plating resists are removed.
In order to achieve the object, a method of manufacturing a printed wiring board according to a fourth aspect of the present invention comprises: a first step in which a laminate including an insulating layer and a plurality of wiring layers is obtained, the insulating layer having a first surface and a second surface located on the opposite side of the first surface, and the plurality of wiring layers being arranged on the first and second surfaces of the insulating layer and inside thereof so as to correspond to a predetermined circuit pattern; a second step in which a via whose one end is opened on the first surface of the insulating layer and the other end of which is closed by the wiring layer inside of the insulating layer is formed on the laminate; a third step in which a first plating resist covers the second surface of the insulating layer and a region other than a portion on which one end of the via is opened out of the first surface of the insulating layer as well as the wiring layers located on these first and second surfaces; a fourth step in which a first plating layer continuously covers the inner surface of the via, the wiring layer inside of the insulating layer exposed within the via and a portion where the plating resist comes out of the wiring layer located on the first surface; a fifth step in which a second plating layer is laminated on the first plating layer, the wiring layer located on the first surface and the wiring layer inside of the insulating layer are electrically connected through the first and second plating layers; and a sixth step in which after an electrical connection is completed between the wiring layers, the plating resists are removed.
According to such a method of manufacture, at the time when the first and second plating layers are formed on the inner surface of the via, the first and second surfaces of the insulating layer as well as the wiring layers located on these surfaces are covered with the plating resist. Therefore, the first and the second plating layers are not doubly adhered to the wiring layers located on the first and second surfaces. Moreover, the other end of the via is closed by the wiring layer. Therefore, the first and second plating layers remain inside of the via and not exposed on the second surface of the insulating layer.
Therefore, the wiring layers located on the first and second surfaces of the insulating layer can be made thinner and a pitch and a pattern width of these wiring layers are easily made finer.
Moreover, in the case where the insulating layer is made, for example, of a flexible material such as a resin film or the like, the following advantage exists: since the first and second plating layers are not necessary to be laminated on the wiring layers, by employing a rolled steel plate as a metal foil configuring this wiring layer, even if the bending force is repeatedly added to the printed wiring board or even if it is the usage form such that it is bended in a U shape, a break of the wiring layers due to the missing of the plating layers can be prevented. Therefore, the reliability of electrical connection is enhanced as well as the life of the printed wiring board becomes longer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.